1. Field of the Invention
The present invention generally relates to the location finding of emf radiation emitters and particularly to methods and apparatus for such which measure signal phase variation during relative movement.
2. Statement of the Prior Art
The proliferation of cellular telephone, or cell phone, technology and usage has revealed various applications for the ability to locate cell phones. These applications include "911" calls and the tracking of unauthorized cell phone usage and illegal activities in which cell phones are used. One of the problems which limits success in this area is multipath reflections of cell phone signals. This problem is acute in urban environments where there is greater fraudulent use of cell phone accounts and especially for illegal activities. The multipath problem is further compounded when a cell phone is being used within a large building and there is a need to locate and/or apprehend the user.
A large amount of technology exists for locating the source of radio emissions using direction finding techniques. These techniques are very effective in open environments where there are few if any multipath reflections. Unfortunately, multipath reflections interfere with the original electromagnetic wavefront and create a resulting wavefront which changes direction in an unpredictable manner. In such cases, direction finding equipment which detects the normal direction to a wavefront is easily confused. Likewise, signal strength and doppler measurements are ambiguous.